The present invention generally relates to communications systems and, more particularly, to satellite-based communications systems.
As described in U.S. Pat. No. 5,966,412 issued Oct. 12, 1999 to Ramaswamy, a form of multi-level modulation such as hierarchical modulation can be used in a satellite system as a way to continue to support existing legacy receivers yet also provide a growth path for offering new services. In other words, a hierarchical modulation based satellite system permits additional features, or services, to be added to the system without requiring existing users to buy new satellite receivers. In a hierarchical modulation based communications system, at least two signals, e.g., an upper layer (UL) signal and a lower layer (LL) signal, are added together to generate a synchronously modulated (i.e., symbol-synchronized) satellite signal for transmission. In the context of a satellite-based communications system that provides backward compatibility, the LL signal provides additional services, while the UL signal provides the legacy services, i.e., the UL signal is, in effect, the same signal that was transmitted before—thus, the satellite transmission signal can continue to evolve with no impact to users with legacy receivers. As such, a user who already has a legacy receiver can continue to use the legacy receiver until such time that the user decides to upgrade to a receiver, or box, that can recover the LL signal to provide the additional services.
In a similar vein, another form of multi-level modulation—layered modulation—can also be used to provide an approach that is backward compatible. In a layered modulation based system at least two signals are modulated (again, e.g., a UL signal (legacy services) and an LL signal (additional services)) onto the same carrier (possibly asynchronously with each other). Transmission of the UL signal and the LL signal occur separately via two transponders and the front end of a layered modulation receiver combines them before recovery of the data transported therein.